The invention involves a heat exchanger for the thermal conditioning of mixtures of substances or for the sterilization of liquids that are or may be contaminated with microorganisms, having an intake flange and an outlet flange and a tube bundle that connects these two flanges, which is sealed inside a shell having ports for the feeding-in and removal of the heating medium. The invention also involves a process for the thermal conditioning of mixtures of substances or for the sterilization of liquids.
Heat exchangers of this type are used, for instance, in the food processing industry, the pharmaceutics industry, and in biotechnology fields, as well as in other areas of process engineering in which liquid media must be heated to high temperatures in the shortest time possible. This heating results in the sterilization of these liquids by killing off undesirable microorganisms and microbes. One problem, however, is that heat labile components and useful materials, such as vitamins and proteins, also become denatured in the process of heat treatment, with the duration of the heat treatment being of primary importance in terms of this negative effect. This so-called denaturation is a particular problem in what is termed the discontinuous sterilization process, which generally involves long heating-up, residence, and cooling times. An additional disadvantage to the process of discontinuous sterilization is that the packaging must also be heated for sterilization. For this reason, continuous sterilization, in which short residence times are possible, is preferred. In the food processing industry, the ultra-high temperature processing of milk is a particularly well-known example of this.
For this known process of continuous sterilization, parallel-plate heat exchangers are generally used in industry. These are comprised of plates that are layered one on top of another and contain special, waved indentations that form the flow canals. These plates are generally pressed together in large numbers by means of tension rods between thick-walled holding plates, and support one another, according to the shape of the waves, at several points. The distance between the plates ranges from 2.5 to 12 mm, which creates correspondingly varied flow canal sizes. The product of value and the heat-exchanging medium flow alternatingly between every two plates. Depending upon the design, the flow paths for the product of value in the individual canals, from the intake opening to the outlet opening, are of varying lengths in all design types, regardless of whether the overflow of the plates is diagonal or curved. This necessarily creates a correspondingly broad residence time distribution for the products to be sterilized using these known parallel-plate heat exchangers, with the result that a certain portion of the heat-sensitive components, for which the period spent in the heat exchanger lies above the average residence time, are subjected to severe denaturation. An even wider range of residence times is created by the hydraulic boundary layers or dead areas that are created at the points of contact of adjacent plates, in which the rate of flow naturally drops to very low levels.
In general technology, tubular heat exchangers are known; these advantageously contain large flow areas and flow paths that are equal in length. Liquids that flow through these tubes have an equal distribution of residence times. The disadvantage of these heat exchangers, however, is that due to the correspondingly varied feeding-in of the medium from the central feed tube, radically dissimilar flow paths and varied residence times are created. In addition, these known tubular heat exchangers are unsuitable for the sterilization of liquids or for the conditioning of mixtures of substances because they do not permit short-time sterilization within the range of seconds.